It is well known that power semiconductor devices are the building blocks of power electronic products, which have been developed rapidly due to the intensive demands in the recent years. With the development of semiconductor technology, the size of power semiconductor devices continues to shrink; their weights become lighter; the safety and reliability of these devices are higher; and the power consumption is lower. Power semiconductor devices, also called power electronic devices, include discrete power semiconductor devices and integrated power semiconductor circuits. These devices have been widely used to convert and control current, voltage, frequency, phase and phase number to achieve functions of a rectifier (AC-to-DC), inverter (DC-to-AC), chopper (DC-to-DC), switch, and/or amplifier, etc. And they are able to tolerate high voltages and withstand high currents.
The power semiconductor devices are generally used to function as switches, because they have minimum power consumption at “ON” and “OFF” states. Therefore, power semiconductor devices constitute the heart of electronic systems. They have been widely used in the fields of communication, consumer electronics, automobile, and industry control, etc.
Power semiconductor devices can be categorized into diodes, triodes, power transistors, and thyristors according to their device structures. Power transistors are becoming dominant in power semiconductor devices because of their excellent performance. Power transistors can be classified as metal-oxide semiconductor field effect transistors (MOSFETs) and insulated gate biploar transistors (IGBTs). Power MOSFETs are provided with two types of device structures: vertical double-diffused MOSFETs (VDMOSFETs) and lateral double-diffused MOSFETs (LDMOSFETs). Because LDMOSFETs occupy larger chip areas than VDMOSFETs and cannot have a higher chip-integration level, VDMOSFETs have become the trend of power transistor research and development.
In principle, a VDMOSFET is the combination of a MOSFET and a junction gate field-effect transistor (JFET), where the N drift region is equivalent to the channel region of a JFET. Therefore, the width, doping concentration and inner defects of the N drift region can significantly affect the device performance. For example, if the resistivity of the N drifts region is high and a portion of the region underneath P region is not conductive, the on-resistance may be still high, which would affect the output power. Similarly, voltage tolerance and surface breakdown of P—N junction may also significantly affect the device performance.
Two major parameters, breakdown voltage (VBD) and on-resistance (RON), must be seriously considered when designing power MOSFETs. Breakdown voltage decreases dramatically when the resistivity of depletion layer (epitaxial layer) decreases, and on-resistance depends on the resistivity of epitaxial layer. The higher doping concentration of depletion layer is, the lower on-resistance (RON) is, which also results in a lower breakdown voltage (VBD). And vice versa, the lower the doping concentration is, the higher on-resistance (RON) is, although decreasing the carrier number in depletion layer would result in a higher breakdown voltage (VBD). Thus, these two parameters appear contradictory, which means that if a higher breakdown voltage is needed, the on-resistance will be higher too; if a lower on-resistance is preferred, the breakdown voltage will be lower.
Thus, there is a limitation concerning the dependence of on-resistance on breakdown voltage, called “silicon limit”, under which on-resistance cannot be lowered further. To break this limitation, a theory called “super junction theory” was developed, which substitutes the low-level doped drift region in conventional power MOSFETs with numerous alternatively distributed P pillars and N pillars as voltage support layers. MOSFETs designed by this theory are called super junction (SJ) MOSFETs. The P pillars in SJMOSFETS have relatively large depth-to-width ratios, which means that the depth of P pillars is larger than their width.
In an SJMOSFET, the epitaxial layer on the substrate is replaced by alternatively distributed N-type drift regions and P-type isolation regions. P-type isolation regions are placed in adjacent N-type drift regions to form P—N junctions. When the SJMOSFET is at on-state, drift current passes through the N-type drift regions. On the other hand, when an SJMOSFET is at cut-off state, depletion layer spreads from every P—N junction between N-type drift region and P-type isolation region to N-type drift region. Under this circumstance, the outermost part of the laterally spreading depletion region which passes through both sides of vertical direction of P-type isolation region speeds up the depletion, which depletes P-type isolation region simultaneously. Therefore, the breakdown voltage (VBD) increases. In addition, the on-resistance of MOSFET can be reduced by increasing the doping level of N-type drift regions.
Although SJMOSFETs have great advantages compared to conventional MOSFETs, the fabrication process is difficult due to the difficulties for forming alternatively distributed P-type and N-type semiconductor regions. Currently, there are two major fabrication processes to form these structures. One is multi-epitaxial growth, which uses multiple times of epitaxial growth to form N-type drift regions. The other is to deposit a thick N-type epitaxial layer, followed by etching deep trenches, then to fill the trenches using P-type silicon epitaxial growth to form the SJMOSFET with alternatively distributed P—N junctions. The first method is easier than the second one, but with a higher cost. The second method is relatively difficult, especially for the trench filling process.
Existing methods mainly utilize a mixture of halogenoid gas and silicon source gas to fill trenches. However, if a silicon source gas is used alone, voids may form inside of the trenches. These voids will reduce breakdown voltage of power semiconductor devices and deteriorate device electric properties. If the mixture of two kinds of gases, halogenoid gas and silicon source gas, is used, the trench filling time may be longer. Therefore, how to ensure the trench filling effects while still retaining desired trench filling efficiency, i.e., the time to fill the trenches is not too long, is becoming a challenging technological topic in the field of design and fabrication of high-power semiconductor devices. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.